(Nasa-Cr-199555) X-Rays from the Youngest Stars
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NASA^CR-199555 (NASA-CR-199555) X-RAYS FROM THE N96-13297 YOUNGEST STARS (Pennsylvania State Univ.) 5 p Unclas 63/93 0072805 X-RAYS FROM THE YOUNGEST STARS Eric D. FEIGELSON Department of Astronomy & Astrophysics, Pennsylvania State University, University Park PA 16802 USA Abstract: The X-ray properties of classical and weak-lined T Tauri stars are briefly reviewed, empha- sizing recent results from the ROSAT satellite and prospects for ASCA. The interpretation of the high level of T Tauri X-rays as enhanced solar-type magnetic activity is discussed and criticized. The census of X-ray emitters is significantly increasing estimates of galactic star formation efficiency, and X-ray emission may be important for self-regulation of star forma- tion. ASCA images will detect star formation regions out to several kiloparsecs and will study the magnetically heated plasma around T Tauri stars. However, images will often suffer from crowding effects. 1. Introduction In a series of seminal studies, Hayashi (1966) calculated that a solar-mass young star de- scends almost vertically down the Hertzsprung-Russell diagram with a fully convective interior, whence it moves to the left to reach the zero-age main sequence. Recent numerical models confirm this basic result. This evolution, however, does not treat the circumstellar matter, which can affect or even dominate the radiative emission of the young stellar object. Insight into the star's environment as it descends its Hayashi track has emerged principally from in- frared, millimeter and radio studies. Shu et al. (1987) incorporate the optical and infrared results into a four-stage scenario for star formation: (a) the molecular cloud cores, commencing gravitational collapse mediated through ambipolar diffusion; (b) protostars with gas infalling onto a circumstellar disk; (c) protostars where a bipolar outflow coexists with the inflow; and (d) classical T Tauri stars, where infall has terminated, revealing an optically visible stars interacting with its accretion disk. To this, one can add a fifth stage: (e) a post T Tauri star that has lost its disk, as the star moves toward the zero-age main sequence. These stages have characteristic intensities and spectra in the mid-infrared bands: stages (b) through (e) correspond to Class 0 through 3 infrared sources (Andre & Montmerle 1994). All of the processes outlined above involve relative cool thermal gases with temperatures ranging from 10 K for the molecular clouds to 3000 K for the T Tauri photospheres. Main sequence stars do emit X-rays from-.magnetically confined hot plasma in coronae and flares. But main sequence stars at the distance of star forming regions would produce only < 10~15 erg/s/cm2 flux in the soft X-ray band. It was thus a considerable surprise that early Einstein Observatory X-ray images of nearby star forming clouds revealed dozens of compact sources as- sociated with low mass pre-main sequence stars with fluxes around 10~13 erg/s/cm2. ROSAT observations with more sensitivity and higher spatial resolution are finding hundreds of new of X-ray emitting T Tauri stars. Furthermore, Tenma and Ginga satellite observations of these regions, while unable to resolve them into individual sources, establishes that the hard X-ray emission is yet another component of T Tauri emission, in some cases order of magnitude stronger than that seen in the soft X-ray band. Extensive research has been conducted to investigate this excess X-ray emission from T Tauri stars, reviewed in detail with many references by Bertout (1989), Feigelson et al. (1991) and Montmerle et al. (1993). We can mention only a few results here, concentrating on the most recent findings. 2. X-ray Properties of T Tauri Stars X-ray studies of pre-main sequence stars have concentrated on the nearest star forming clouds about 150 pc away, lying just outside the local 'hot bubble' surrounding the Sun. These regions include the Taurus-Auriga complex covering several hundred square degrees in the northern sky, and smaller clouds in the southern sky such as the Ophiuchi, Corona Australis, Chamaeleon and Lupi clouds (see map in Dame et al. 1987). The Orion molecular cloud, about 450 pc distant, is also extremely rich in X-ray sources, but the X-ray fluxes are reduced and crowding is worse. A soft X-ray image of these nearby star forming regions typically shows dozens of faint X-ray sources associated with stars in and around the molecular cores. Many of these are previously studied 'classical' T Tauri (CTT) stars, with strong broad optical emission lines attributed to an outflowing wind or accreting gas and strong infrared excesses produced by a cool dusty circumstellar disk. However, the majority of X-ray selected stars are 'weak-lined' T Tauri stars (WTT) which exhibit only a weak variable Ha line and little or no infrared excess. Initially these were thought to be the missing post-T Tauri stars (stage e above), but many of the X-ray WTT stars are as luminous (hence as young) as CTT stars. CTT and WTT stars have similar X-ray luminosity functions, ranging from several x 1030 to a sensitivity limit of several x 1028 erg/s in the soft X-ray band. Repeated observations show that virtually all CTT and WTT are variable in X-rays on timescales of days. Perhaps one percent of these stars are undergoing a powerful flare up to 1031 erg/s on timescales of minutes or hours during any single observation. The X-ray spectra of most T Tauri tars can be modeled in terms of a soft (0.4 keV) and a harder (1.5 keV) component, with different amounts of foreground absorption depending on the star's position in the molecular cloud. However, Tenma and Ginga non-imaging observations of the p Ophiuchi cloud show that some T Tauri stars emit very strongly in the 2-10 keV band with an iron line at 6.7 keV (Koyama 1992). Variability in this hard band suggests that only one or a few stars are responsible for this component, although its power may exceed that from all of the soft X-ray emitters combined. Observations now emerging from ROSAT pointed observations with the Position Sensitive Proportional Counter are quite spectacular. A mosaic 6 ksec exposures of the Chamaeleon I cloud shows 89 sources, of which 20-35 are previously unreported WTT stars (Feigelson et al. 1993). A much longer exposure sees few additional sources, suggesting that the bulk of the T Tauri population is now seen (Krautter et al. 1994). A very deep image of the L1495E cloud in Taurus-Auriga reveals a new group of low mass T Tauri M stars, giving an initial mass function compatible with that seen in the solar neighborhood (Strom & Strom 1994). The #OS.AT-discovered stars may soon outnumber all previously known T Tauri stars. A mosaic of 17 pointings towards the Orion molecular cloud has over 600 X-ray sources (Walter 1994), and preliminary results from optical followup of the ROSAT all-sky survey indicate that as many as 450-500 new WTTs may be found in the Taurus-Auriga clouds (Neuhauser et al. 1994). In addition to surveys for new WTT stars, important individual stars have been studied in detail. A simultaneous .ROS-AT/radio/optical campaign of the luminous WTT star HD 283447 showed strong but constant X-ray and chromospheric activity simultaneous with rapidly variable radio emission (Feigelson et al. 1994). 3. The Solar Activity Interpretation These X-ray properties bear a significant resemblance to those seen in magnetically active late-type stars such as dMe flare stars, tidally spun-up RS CVn binary systems, and the Sun. In each case, plasma is heated to X-ray temperatures by continuous and/or explosive processes, and is trapped in in a complex multipolar configurations of closed magnetic structures attached to the stellar surface. If the X-ray emission of T Tauri stars arises from solar-type magnetic activity, then the total plasma emission must be 2-4 orders of magnitude greater than in the Sun. Simple cooling loop models of X-ray flares in T Tauri stars, with peak luminosities around 31 Lr(0.5-4.5 keV) ~ 10 erg/s and decay times of hours, suggest that the plasma densities are similar to solar values but the loop sizes are much larger around 1011 cm. Similar X-ray flares and emission measures are seen from RS CVn systems. Considerable evidence from other spectral bands has emerged supporting the enhanced solar activity model of T Tauri X-ray emission (see Feigelson et al. 1991 for a detailed review). Optical photometric monitoring of X-ray luminous WTT stars often show rotational modula- tion of cool spots covering 5-40% of the stellar surface. In one WTT star, rapid photometry caught a white light continuum flare causing At/ — 0.4 magnitudes in 20 minutes. In many cases, optical and ultraviolet spectroscopy reveals greatly enhanced chromospheric emission lines. This has been known for several decades and led to the 'deep chromosphere' model for WTT stars. Radio continuum flares with power up to 1018 erg/s/Hz, or 105 times peak solar levels, are seen in some WTT stars. The radio spectra are flat or inverted, and circular polarization is seen in a few cases which demonstrates that solar-type gyrosynchrotron emission is present. Very Long Baseline Interferometry has resolved these radio emitting regions in the brightest stars, showing variable sizes from less than a stellar radius to 5-20 R* (Phillips et al. 1991). (Recall that along the Hayashi track stars typically have radii R,, ~ 2 — S.RQ .) This observationally supports earlier inferences from the X-ray flares that the magnetic structures in WTT stars are orders of magnitude larger than in the Sun.